This invention relates to a spark plug.
A spark plug used for ignition of an internal engine of such as automobiles generally comprises a metal shell to which a ground electrode is fixed, an insulator made of alumina ceramics, and a center electrode which is disposed inside the insulator. The insulator projects from the rear opening of the metal shell in the axial direction. A terminal metal fixture is inserted into the projecting part of the insulator and is interconnected to the center electrode via a conductive glass seal layer which is formed by a glass sealing procedure or a resistor. A high voltage is applied to the terminal metal fixture to cause a spark over the gap between the ground electrode and the center electrode.
Under some combined conditions, for example, at an increased spark plug temperature and an increased environmental humidity, it may happen that high voltage application fails to cause a spark over the gap but, instead, a discharged called as a flashover occurs between the terminal metal fixture and the metal shell, going around the projecting insulator. Primarily for the purpose of avoiding flashover, most of commonly used spark plugs have a glaze layer on the surface of the insulator. The glaze layer also serves to smoothen the insulator surface thereby preventing contamination and to enhance the chemical or mechanical strength of the insulator.
In the case of the alumina insulator for the spark plug, such a glaze of lead silicate glass has conventionally been used where silicate glass is mixed with a relatively large amount of PbO to lower a dilatometric softening point. In recent years, however, with a globally increasing concern about environmental conservation, glazes containing Pb have been losing acceptance. In the automobile industry, for instance, where spark plugs find a huge demand, it has been a subject of study to phase out Pb glazes in a future, taking into consideration the adverse influences of wasted spark plugs on the environment.
Leadless borosilicate glass- or alkaline borosilicate glass-based glazes have been studied as substitutes for the conventional Pb glazes, but they inevitably have inconveniences such as a high glass viscosity or an insufficient insulation resistance. In particular, in the case of the glaze for spark plugs, since being served together with engines, it more easily increases temperature than ordinary insulating porcelains (maximum: around 200xc2x0 C.), and recently being accompanies with high performance of engines, voltage to be supplied to the spark plug has been high, and the glaze has been demanded to have the insulating performance durable against more severer. Actually, for restraining the flashover under a condition of increasing temperature, such a glaze is necessary which is more excellent in the insulating property under the condition of increasing temperature.
In the existing leadless glaze for spark plugs, for checking a melting point from going up effected by removing a lead component, an alkaline metal component has been mixed. The alkaline metal component is effective for securing fluidity when baking the glaze. However, the more the content of the alkaline metal component, the lower the insulating resistance of the glaze, and an anti-flashover property is easily spoiled. Therefore, the alkaline metal component in the glaze should be limited to a necessary minimum for increasing the insulating property.
So, the existing leadless glaze has inevitably wanted the content of the alkaline metal, a vitreous viscosity is likely to increase at high temperature (when melting the glaze) in comparison with a Pb-glaze, and after baking the glaze, there easily appear pinholes or glaze crimping. For removing these defects, it is assumed to heighten the glaze baking temperature so as to improve the fluidity, but the heightening of the glaze baking temperature is not preferable since it invites an energy cost-up and to shorten lives of facilities.
It is an object of the invention to offer such a spark plug which contains a smaller Pb component, is excellent in the fluidity when baking the glaze, high in the insulating resistance, and good in the anti-flashover.
The spark plug according to the invention has a structure having an alumina ceramic insulator disposed between a center electrode and a metal shell, wherein at least part of the surface of the insulator is covered with a glaze layer of oxide being a main.
In this first structure, the glaze layer is characterized by comprising
Pb component 1 mol % or less in terms of PbO;
Si component 30 to 60 mol % in terms of SiO2;
B component 20 to 50 mol % in terms of B2O3;
Zn component 0.5 to 25 mol % in terms of ZnO;
Ba and/or Sr components 0.5 to 15 mol % in terms of BaO or SrO in total;
alkaline metal components of 2 to 12 mol % in total of
two kinds or more of Na in terms Na2O, K in terms of K2O and Li in terms of Li2O, K and Li being essential, respectively; and
F component 0.1 to 10 mol % in terms of F2.
In a second structure, the glaze layer is characterized by comprising
Pb component 1 mol % or less in terms of PbO;
Si component 30 to 60 mol % in terms of SiO2;
B component 20 to 40 mol % in terms of B2O3;
Zn component 0.5 to 25 mol % in terms of ZnO;
Ba and/or Sr components 0.5 to 15 mol % in terms of BaO or SrO in total;
alkaline metal components of 2 to 12 mol % in total of
one kind or more of Na in terms Na2O, K in terms of K2O and Li in terms of Li2O, respectively;
F component 0.1 to 10 mol % in terms of F2; and
one kind or more selected from Bi, Sb and rare earth elements RE (selected from a group of Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu) of 0.1 to 5 mol % in total of Bi in terms of Bi2O3, Sb in terms of Sb2O5, as to RE, Ce in terms of CeO2, Pr in terms of Pr7O11, and others in terms of RE2O3.
In the spark plug according to the invention, for aiming at the adaptability to the environmental problems, it is a premise that the glaze to be used contains the Pb component 1.0 mol % or less in terms of PbO (hereafter called the glaze containing the Pb component reduced to this level as xe2x80x9cleadless glazexe2x80x9d). When the Pb component is present in the glaze layer in the form of an ion of lower valency (e.g., Pb2+), it is oxidized to an ion of higher valency (e.g., Pb3+) by a corona discharge. If this happens, the insulating properties of the glaze layer are reduced, which probably spoils an anti-flashover. From this viewpoint, too, the limited Pb content as mentioned above is beneficial. A preferred Pb content is 0.1 mol % or less. It is most preferred for the glaze to contain substantially no Pb (except a trace amount of lead unavoidably incorporated from raw materials of the glaze).
While lowering the Pb content as mentioned above, the invention selects the above mentioned particular compositions for providing the insulating performance, optimizing the glaze baking temperature (actually, lowering temperature) and securing a good glaze-baked finish. In the existing glaze, the Pb component plays an important part as to adjustment of the dilatometric softening point (practically, appropriately lowering the dilatometric softening point of the glaze and securing the fluidity when baking the glaze) but in the leadless glaze, the B component (B2O3) and the alkaline metal have a deep relation with adjustment of the dilatometric softening point. Inventors found that the B component has a particularly convenient range for improving the glaze baking finish in relation with the content of the Si component, and if the F component is contained in the above mentioned range, the fluidity when baking the glaze may be secured while controlling the content of the alkaline metal to be relatively low, and in turn the baking of the glaze is possible at relatively low temperatures, the glaze layer having an excellent and smooth baked surface is available, and they completed this invention.
Detailed explanation will be made to roles and critical significances of the respective components (the explanation is common to the first and second structures, excepting especial remarks).
The alkaline metal component is inherently high in ion conductivity and trends to lower the insulating property in the glaze layer of vitreous substance. On the other hand, the Si component or the B component form a vitreous skeleton, and by appropriately determining the contents, sizes of network of skeleton are made suitable for blocking the ion conductivity of the alkaline metal and securing the desirable insulating property. Since the Si component or the B component are ready for forming skeleton, they trend to lower the fluidity when baking the glaze, but by containing the alkaline metal component of the appropriate amount together with the components of improving fluidity, the fluidity is heightened by lowering melting points by a eutectic reaction and preventing formation of complex anion by mutual action of Si ion and O ion.
The Si component is difficult to secure the sufficient insulating property if being less than 30 mol %, and is difficult to bake the glaze if being more than 60 mol %. On the other hand, if the B component is less than 20 mol %, the dilatometric softening point of the glaze rises and the baking of the glaze is difficult. An upper limit of the B component is 50 mol % in the first structure and 40 mol % in the second structure. If the B component is contained over these upper limits, the fluidity exceedingly increases, crimping is easily created in the glaze. In the second structure, the increasing of the fluidity is prospected by an amount of containing a fluidity improving component though the B component is lower than that of the first structure. Accordingly, the upper limit of the B component is determined to be lower than that of the first structure in response to the minimum addition amount (0.1 mol %) of the fluidity improving component. If the B content exceeds the upper limits, depending on contents of other components, there probably occur problems about devitrification of the glaze layer, decrease of the insulating property or non-compatibility with thermal expansion coefficient.
If the Zn component is less than 0.5 mol %, the thermal expansion coefficient of the glaze layer is too large, defects such as crazing easily occur in the glaze layer. Since the Zn component also acts to lower the dilatometric softening point of the glaze, if it is short, the baking of the glaze will be difficult. Being more than 25 mol %, opacity easily occurs in the glaze layer due to the devitrification. It is good that the Zn containing amount to determine 10 to 20 mol %. When containing the Zn component within this desirable range, the fluidity improving effect can be also expected by lowering of the dilatometric softening point of the Zn component itself, and in this case, the total amount of the fluidity improving components is desirably 0.1 to 2.5 mol %.
The Ba or Sr components contribute to heightening of the insulating property of the glaze layer and is effective to increasing of the strength. If the total amount is less than 0.5 mol %, the insulating property of the glaze layer goes down, and the anti-flashover might be spoiled. Being more than 15 mol %, the thermal expansion coefficient of the glaze layer is too high, defects such as crazing easily occur in the glaze layer. In addition, the opacity easily occurs in the glaze layer. From the viewpoint of heightening the insulating property and adjusting the thermal expansion coefficient, the total amount of Ba and Sr is desirably determined to be 0.5 to 10 mol %. Either or both of the Ba and Sr component may be contained, but the Ba component is advantageously less expensive in a cost of a raw material.
The Ba and Sr components may exist in forms other than oxides in the glaze depending on raw materials to be used. For example, BaSO4 is used as a source of the Ba component, an S component might be residual in the glaze layer. This sulfur component is concentrated nearly to the surface of the glaze layer when baking the glaze to lower the surface expansion of a melted glaze and to heighten a smoothness of a glaze layer to be obtained.
The total amount of the Zn component and Ba and/or Sr components is desirably 7 to 25 mol % in terms of oxide. If the total amount exceeds 25 mol %, the glaze layer will be slightly opaque. For example, on the outer surface of the insulator, visual information such as letters, figures or product numbers are printed and baked with color glazes for identifying makers and others, and owing to the slight opaqueness, the printed visual information is sometimes illegible. Or, if being less than 7 mol %, the dilatometric softening point exceedingly goes up to make the glaze baking difficult and cause bad external appearance. Thus, the total amount is more desirably 10 to 20 mol %.
Next, if the total amount of the alkaline metal components
is less than 2 mol %, the dilatometric softening point of the glaze goes up, and the baking of the glaze might be probably impossible. In case of being more than 12 mol %, the insulating property probably goes down, and an anti-flashover might be spoiled. With respect to the alkaline metal components, not depending on one kind, but adding in joint two kinds or more selected from Na, K and Li, the insulating property of the glaze layer is more effectively restrained from lowering. As a result, the amount of the alkaline metal components can be increased without decreasing the insulating property, consequently it is possible to concurrently attain the two purposes of securing the fluidity when baking the glaze and the anti-flashover (so-called alkaline joint addition effect).
In the first structure, as to the alkaline metal components, K and Li are indispensably contained. As the K component has a larger atomic amount than those of Na and Li, in case the total amount of the alkaline metal components is set to be the same mol %, the K component does not exhibit the fluidity improving effect as the Na or Li components, but comparing with Na or Li (particularly, Li), as an ionic migration of K is comparatively small in the glaze layer of the vitreous substance, the K component has an inclination difficult to lower the insulating property of the glaze layer, though increasing the amount. On the other hand, as the Li component has the small atomic amount, the fluidity improving effect is larger than that of the K component, but as the ionic migration is high, an exceeding addition easily brings about reduction of the insulating property of the glaze layer.
Therefore, in the first structure, for always securing the fluidity of a necessarily enough level also in case a later mentioned fluidity improving component is not added, an inclusion of the Li component having a large fluidity improving effect is indispensable, and for compensating reduction of the insulating property by increase of the Li component, an addition of K is made a premise. Of course, at least two kinds of alkaline metal components are added in joint, so that the insulating property improving effect by the joint addition of alkaline metals is accomplished (on the other hand, in the second structure based on the premise of adding the fluidity improving component, no limitation is made to kinds of the alkaline metal components to be contained.)
For example, among the alkaline metal components, it is possible to effectively restrain the insulating property of the glaze layer from lowering by making the amount of the K component highest, and by mixing the Li component of the amount next to the highest amount of K, it is possible to secure the fluidity when baking the glaze, restrain increase of the thermal expansion coefficient of the glaze layer by mixing the K component, and match with the thermal expansion coefficient of alumina in a substrate. The inclination of the insulating property decreasing by addition of the Li component can be effectively restrained by the above mentioned joint addition of alkaline metals by the three components by compounding Na of the smaller amount than those of K or Li. As a result, it is possible to realize such a glaze composition which is high in the insulating property, rich in the fluidity when baking the glaze, and small in difference between the thermal expansion coefficients with that of alumina being the insulator composing ceramic.
Specifically, it is desirable to set the rate of the K component of the alkaline metal components of Na, K and Li in the mol % in terms of oxide as
0.4xe2x89xa6K/(Na+K+Li)xe2x89xa60.8.
If the value of K/(Na+K+Li) is less than 0.4, the insulating property improving effect by the K addition might be insufficient. On the other hand, that the value of K/(Na+K+Li) is less 0.8 denotes that alkaline metal components other than K are added in joint within a range of a rest being 0.2 or more (0.6 or less), and it is possible to heighten the insulating property by the above mentioned joint addition of alkaline and in turn to improve the anti-flashover. Incidentally, it is desirable to adjust the value of K/(Na+K+Li) to be 0.5 to 0.7.
The Li component is preferred to be contained in order to realize the effect of adding in joint alkaline components for increasing the insulating property, and in order to adjust the heat expansion coefficient of the glaze layer, to secure the fluidity when baking the glaze, and further to increase the mechanical strength. It is preferable that the Li component is contained in the mol amount in terms of oxide in the following range:
0.2xe2x89xa6Li/(Na+K+Li)xe2x89xa60.5.
If the rate of Li is less than 0.2, the heat expansion coefficient becomes too large in comparison with the alumina substrate. As a result, the crazing maybe easily produced to make the baked surface finish of the glaze insufficient. On the other hand, if the rate of Li component exceeds 0.5, this may give an adverse influence to the insulating property of the glaze layer because the Li ion has a comparatively high degree of immigration among the alkaline metal ions. It is preferable that the value of Li/(Na+K+Li) is adjusted in the range of 0.3 to 0.45.
Next, if the F component adds together with the alkaline metal components, it exhibits effects of lowering the dilatometric softening point of the glaze and improving the fluidity when baking the glaze, though controlling the content of the alkaline metal component to be low. If the content is less than 0.1 mol % in terms of F2, the fluidity improving effect is insufficient, and if being more than 10 mol %, air bubbles are ready for arising which are likely to cause breakdown in the glaze when baking it, and this attributes to spoiling of the strength of the insulator having the glaze layer thereon, for example, the impact resistance, and the glaze layer is likely to devitrify owing to much bubbles. Further, a gas containing the F component is issued when baking the glaze, and this trends to invite inconveniences of reacting with a refractory composing an oven wall to shorten the life of the oven wall. The F component is preferably contained 2 to 6 mol % in terms of F2.
Further, it is desirable to adjust the fluidity improving effect by the F addition in response to the addition amount of the alkaline metal components. Specifically, if the total mol containing rate (mol %) in terms of oxide of the alkaline metal components is NR (mol %) and the mol containing rate of the F component in terms of F2 is NF, preferably NF/NR is 0.07 to 1.5. If being less than 0.07, the fluidity improving effect by the F addition is insufficient, and if being more than 1.5, a remarkable heightening of the fluidity improving effect by increasing the F addition is not prospective, and futility is much.
By the way, the F component can be added by compounding a part of a source of a cation component of the glaze layer in a form of fluoride of this cation, for example, in the form of fluoride of Si, alkaline metal, alkaline earth metals, or rare earth metals (actually, LiF or CaF2, provided that the containing rates of the cation components added in the form of fluoride are shown in terms of oxides in this invention). As the fluoride of silicon, for example, silicon fluoride based high polymer can be employed. F compounds dissolved or exhausted in forms of gas of components other than F when preparing the glaze frit, can be added, for example, in a form of fluoride of carbon (polytetrafluoroethylene or graphite fluoride).
Next, in the second structure, the above mentioned fluidity improving components are indispensably contained. Each of these fluidity improving components has effects of heightening the fluidity when baking the glaze, controlling the bubble forming in the glaze layer, or wrapping adhered substances to the glaze baked surface to prevent abnormal projections. Sb and Bi are especially remarkable in these effects (Bi has possibility to be designated as a limited substance in a future). The improvement of the fluidity when baking the glaze is more remarkable by combining two kinds or more of these fluidity improving components. Since the rare earth component comparatively takes cost for separation and refinement, use of non-separating rare earth elements (in this case, those are the composition particular to raw ores and a plurality of kinds of rare earth elements are mixed) is advantageous for saving cost. If the total amount in terms of oxides of the indispensable fluidity improving components is less than 0.1 mol %, there will be probably a case of not always providing an effect of improving the fluidity when baking the glaze for easily obtaining a smooth glaze layer. On the other hand, if exceeding 5 mol %, there will be probably a case of being difficult or impossible to bake the glaze owing to too much heightening of the softening point of the glaze.
If parts of Sb, Bi and the rare earth components are more than 5 mol % in the addition amount, the glaze layer might be excessively colored. For example, visible information such as letters, figures or product numbers are printed with color glazes on external appearances of the insulators for specifying producers and others, and if the colors of the glaze layer is too thick, it might be difficult to read out the printed visible information. As another realistic problem, there is a case that tint changing resulted from alternation in the glaze composition is seen to purchasers as xe2x80x9cunreasonable alternation in familiar colors in external appearancexe2x80x9d, so that an inconvenience occurs that products could not always be quickly accepted because of a resistant feeling thereto.
The insulator forming a substrate of the glaze layer is composed of alumina based ceramics in white, and in view of preventing or restraining coloration, it is desirable that the coloration in an observed external appearance of the glaze layer formed in the insulator is adjusted to be 0 to 6 in chroma Cs and 7.5 to 10 in lightness Vs, for example, the amount of the above transition metal component is adjusted. If the chroma exceeds 6, discrimination by naked eye is conspicuous, and if lightness is 7.5 or lower, the gray or blackish coloration is easily distinguished. In either way, there appears a problem that an impression of xe2x80x9capparent colorationxe2x80x9d cannot be wiped out. The chroma Cs is desirably 0 to 2, more desirably 0 to 1, and the chroma is preferably 8 to 10, more preferably 9 to 10. In the present specification, a measuring method of the lightness Vs and the chroma Cs adopts the method specified in xe2x80x9c4.3 A Measuring Method of Reflected Objectsxe2x80x9d of xe2x80x9c4. Spectral Colorimetryxe2x80x9d in the xe2x80x9cA Measuring Method of Colorsxe2x80x9d of JIS-Z8721. As a simple method, the lightness and the chroma can be known through visual comparisons with standard color chart prepared according to JIS-Z8721.
In the following description, explanation will be made to other components which can be contained in the glaze layer. At first, as auxiliary fluidity improving components, one kind or more of Mo, W, Ni, Co, Fe and Mn are contained 0.5 to 5 mol % in total in terms of MoO3, WO3, Ni3O4, Co3O4, Fe2O3 and MnO2, respectively. If being less than 0.5 mol %, an effect is insufficient, while being more than 5 mol %, the dilatometric softening point of the glaze exceedingly goes up, and the glaze-baking is difficult or impossible. Among the auxiliary fluidity improving components, the most remarkable fluidity improving effects are Mo and Fe, and next is W.
As each of these auxiliary fluidizing improving components is transition element, an excessive addition contributes to inconvenience of causing unintentional coloring in the glaze layer (this might be a problem when using the rare earth element as the fluidity improving component).
It is possible to contain one kind or more of Ti, Zr and Hf 0.5 to 5 mol % in total in terms of ZrO2, TiO2 and HfO2.
By containing one kind or more of Ti, Zr or Hf, a water resistance is improved. As to the Zr or Hf components, the improved effect of the water resistance of the glaze layer is more. noticeable. By the way, xe2x80x9cthe water resistance is goodxe2x80x9d is meant that if, for example, a powder like raw material of the glaze is mixed together with a solvent as water and is left as a glaze slurry for a long time, such inconvenience is difficult to occur as increasing a viscosity of the glaze slurry owing to elusion of the component. As a result, in case of coating the glaze slurry to the insulator, optimization of a coating thickness is easy and unevenness in thickness is reduced. Subsequently, said optimization and said reduction can be effectively attained. If being less than 0.5 mol %, the effect is poor, and if being more than 5 mol %, the glaze layer is ready for devitrification.
It is possible to contain 0.5 to 15 mol % in total of one kind or more of the Al component 0.5 to 5 mol % in terms of Al2O3, the Ca component 0.5 to 10 mol % in terms of CaO, and the Mg component 0.5 to 10 mol % in terms of MgO. The Al component has an effect of restraining the devitrification of the glaze layer, the Ca component and the Mg component contribute to improvement of the insulating property of the glaze layer. In particular, the Ca component is effective next to the Ba component or the Zn component for increasing the insulating property of the glaze layer. If the addition amount is less than each of the above mentioned lower limits, the effect is insufficient, while being more than the upper limit of each of the components or the upper limit of the total amount, the dilatometric softening point exceedingly increases and the glaze-baking might be difficult or impossible.
The glaze layer may contain auxiliary components of one kind or more of Sn, P, Cu, and Cr 0.5 to 5 mol % in total as Sn in terms of SnO2, P in terms of P2O5, Cu in terms of CuO, and Cr in terms of Cr2O3. These components may be positively added in response to purposes or often inevitably included as raw materials of the glaze (otherwise later mentioned clay minerals to be mixed when preparing the glaze slurry) or impurities (otherwise contaminants) from refractory materials in the melting procedure for producing glaze frit. Each of them heightens the fluidity when baking the glaze, restrains bubble formation in the glaze layer, or wraps adhered materials on the baked glaze surface so as to prevent abnormal projections. If the addition amount is less than each of the above mentioned lower limits, the effect is insufficient, while being more than the upper limit of each of the components or the upper limit of the total amount, the dilatometric softening point exceedingly increases and the glaze-baking might be difficult or impossible (in particular, CuO and Cr2O3), or insufficient conductivity (in particular, by excessive amount of SnO2) or insufficient water resistance (in particular, by excessive amount of P2O5) of the glaze layer is caused.
In the structure of the spark plug of the invention, the respective components in the glaze are contained in the forms of oxides, and owing to factors forming amorphous and vitreous phases, the existing forms by oxides cannot be often identified. In this case, if the containing amounts of components at values in terms of oxides in the glaze layer fall in the above mentioned ranges, it is regarded that they belong to the ranges of the invention.
Herein, the containing amounts of the respective components in the glaze layer formed on the insulator can be identified by use of known micro-analyzing methods such as EPMA (electronic probe micro-analysis) or XPS (X-ray photoelectron spectroscopy). For example, if using EPMA, either of a wavelength dispersion system and an energy dispersion system is sufficient for measuring characteristic X-ray. Further, there is a method where the glaze layer is peeled from the insulator and is subjected to a chemical analysis or a gas analysis for identifying the composition.
The spark plug having the glaze layer of the invention may be composed by furnishing, in a through hole of the insulator, a pole-like terminal metal fixture as one body with the center electrode or by holding a conductive bonding layer in relation therewith, said metal fixture being separate from a center electrode. In this case, the insulating resistant value can be measured under a condition where an electric conductivity is made between the terminal metal fixture and a metal shell, keeping the whole of the spark plug at around 500xc2x0 C. For securing an insulating endurance at high temperatures, it is desirable that the insulating resistant value is secured 200 Mxcexa9 or higher, desirably 400 Mxcexa9 so as to prevent the flashover.
The measurement may be carried out as follows. DC constant voltage source (e.g., source voltage 1000 V) is interconnected to the side of a terminal metal 13 of the spark plug 100 shown in FIG. 1, while at the same time, the side of the metal shell 1 is grounded, and a current is passed under a condition where the spark plug 100 disposed in a heating oven is heated at 500xc2x0 C. For example, assuming that a current value Im is measured by use of a current measuring resistance (resistance value Rm) at the voltage VS, an insulation resistance value Rx to be measured can be obtained as (VS/Im)xe2x88x92Rm.
The insulator may be composed of the alumina insulating material containing the Al component 85 to 98 mol % in terms of Al2O3. Preferably, the glaze layer has an average thermal expansion coefficient of 5xc3x9710xe2x88x926/xc2x0 C. to 8.5xc3x9710xe2x88x926/xc2x0 C. at the temperature ranging 20 to 350xc2x0 C. Being less than this lower limit of the average thermal expansion, defects such as cracking or graze skipping easily happen in the graze layer. On the other hand, being more than the upper limit, defects such as crazing are likely to happen in the graze layer. The thermal expansion coefficient more preferably ranges 6xc3x9710xe2x88x926/xc2x0 C. to 8xc3x9710xe2x88x926/xc2x0 C.
The thermal expansion coefficient of the glaze layer is assumed in such ways that samples are cut out from a vitreous glaze bulk body prepared by mixing and melting raw materials such that almost the same composition as the glaze layer is realized, and values measured by a known dilatometer method.
The thermal expansion coefficient of the glaze layer on the insulator can be measured by use of, e.g., a laser inter-ferometer or an interatomic force microscope.
The insulator may be formed with a projection radially extending from the outer periphery at the middle portion in the axial direction thereof, and may be formed cylindrically in an outer periphery of the base portion thereof adjacent the rear side with respect to the projection thereof with a forward portion extending toward a forward end of the center electrode in the axial direction. In general, as to automobile engines, a rubber cap is utilized to attach the spark plug to the electric system of engines. In order to heighten the anti-flashover, adhesion between the insulator and the interior of the rubber cap is important. Therefore, the glaze layer desirably is smooth at a maximum height of 7 xcexcm or less in a curve of a surface roughness in accordance to the measurement prescribed by JIS:B0601 at the outer periphery of the base portion.
According to the study by the inventors, it was found that as to borosilicate glass based- or alkaline borosilicate glass based leadless glaze layer, it was important to adjust the film thickness of the glaze layer for obtaining the smooth surface of the glaze layer. Further, it was found that since the outer periphery in the base portion of the insulator main part is required to closely contact the rubber cap, the adjustment of film thickness, if properly conducted, will increase the anti-flashover. In the insulator having the leadless glaze layer, it is desirable to adjust the film thickness of the glaze layer covering the outer periphery in the base portion of the insulator main part within the range of 7 to 50 xcexcm. Thus, the close contact may be obtained between the glaze baked surface and the rubber cap without lowering the insulating property of the glaze layer, and in turn the anti-flashover may be obtained.
In case the thickness of the glaze layer in the insulator is less than 7 xcexcm, it is difficult to form the uniform and smooth glaze baked surface in the leadless glaze layer of the above mentioned composition, and the close contact between the glaze baked surface and the rubber cap is spoiled, so that the anti-flashover is made insufficient. On the other hand, in case the thickness of glaze layer exceeds 50 xcexcm, a cross sectional area of conductivity increases, so that it is difficult to secure the insulating property with the leadless glaze layer of the mentioned composition, similarly, resulting in lowering of the anti-flashover.
For making the thickness of the glaze layer uniform and restraining the glaze layer from excessive (or local) thickness, the addition of Ti, Zr or Hf is useful as mentioned above.
The spark plug of the invention can be produced by a production method comprising:
a step of preparing glaze powders in which the raw material powders are mixed at a predetermined ratio, the mixture is heated 1000 to 1500xc2x0 C. and melted, the melted material is rapidly cooled, vitrified and ground into powder;
a step of piling the glaze powder on the surface of an insulator to form a glaze powder layer; and
a step of heating the insulator, thereby to bake the glaze powder layer on the surface of the insulator.
The powdered raw material of each component includes not only an oxide thereof (sufficient with complex oxide) but also other inorganic materials such as hydroxide, carbonate, chloride, sulfate, nitrate, or phosphate. These inorganic materials should be those of capable of being converted to oxides by heating and melting. The rapidly cooling can be carried out by throwing the melt into a water or atomizing the melt onto the surface of a cooling roll for obtaining flakes.
The glaze powder is dispersed into the water or solvent, so that it can be used as a glaze slurry. For example, if coating the glaze slurry onto the insulator surface to dry it, the coating layer of the glaze powder (the glaze powder layer) can be formed. By the way, as the method of coating the glaze slurry on the insulator surface, if adopting a method of spraying from an atomizing nozzle onto the insulator surface, the glaze powder layer in uniform thickness of the glaze powder can be easily formed and an adjustment of the coated thickness is easy.
The glaze slurry can contain an adequate amount of a clay mineral or an organic binder for heightening a shape retention of the glaze powder layer. As the clay mineral, those composed of mainly aluminosolicate hydrates can be applied, for example, those composed of mainly one kind or more of allophane, imogolite, hisingerite, smectite, kaolinite, halloysite, montmorillonite, illite, vermiculite, and dolomite (or mixtures thereof) can be used. In relation with the oxide components, in addition to SiO2 and Al2O3, those mainly containing one kind or more of Fe2O3, TiO2, CaO, MgO, Na2O and K2O can be used.
The spark plug of the invention is constructed of an insulator having a through hole formed in the axial direction thereof, a terminal metal fixture fitted in one end of the through hole, and a center electrode fitted in the other end. The terminal metal fixture and the center electrode are electrically interconnected in the through hole via an electrically conductive sintered body mainly comprising a mixture of a glass and a conductive material (e.g., a conductive glass seal or a resistor). The spark plug having such a structure can be made by a process including the following steps.
An assembly step: a step of assembling a structure comprising the insulator having the through hole, the terminal metal fixture fitted in one end of the through hole, the center electrode fitted in the other end, and a filled layer formed between the terminal metal fixture and the center electrode, which (filled layer) comprises the glass powder and the conductive material powder.
A glaze baking step: a step of heating the assembled structure formed with the glaze powder layer on the surface of the insulator at temperature ranging 800 to 950xc2x0 C. to bake the glaze powder layer on the surface of the insulator so as to form a glaze layer, and at the same time softening the glass powder in the filled layer.
A pressing step: a step of bringing the center electrode and the terminal metal fixture relatively close within the through hole, thereby pressing the filled layer between the center electrode and the terminal metal fixture into the electrically conductive sintered body.
In this case, the terminal metal fixture and the center electrode are electrically interconnected by the electrically conductive sintered body to concurrently seal the gap between the inside of the through hole and the terminal metal fixture and the center electrode. Therefore, the glaze baking step also serves as a glass sealing step. This process is efficient in that the glass sealing and the glaze baking are performed simultaneously. Since the above mentioned glaze allows the baking temperature to be lower to 800 to 950xc2x0 C., the center electrode and the terminal metal fixture hardly suffer from bad production owing to oxidation so that the yield of the spark plug is heightened. The baking glaze step can be preceded to the glass sealing step.
The dilatometric softening point of the glaze layer is preferably adjusted to range, e.g., 520 to 700xc2x0 C. When the dilatometric softening point is higher than 700xc2x0 C., the baking temperature above 950xc2x0 C. will be required to carry out both baking and glass sealing, which may accelerate oxidation of the center electrode and the terminal metal fixture. When the dilatometric softening point is lower than 520xc2x0 C., the glaze baking temperature should be set lower than 800xc2x0 C. In this case, the glass used in the conductive sintered body must have a low dilatometric softening point in order to secure a satisfactory glass seal. As a result, when an accomplished spark plug is used for a long time under a relatively high temperature environment, the glass in the conductive sintered body is liable to denaturalization, and where, for example, the conductive sintered body comprises a resistor, the denaturalization of the glass tends to result in deterioration of the performance such as a life under load. Incidentally, the dilatometric softening point of the glaze is adjusted at temperature range of 520 to 620xc2x0 C.
The dilatometric softening point of the glaze layer is a value measured by performing a differential thermal analysis on the glaze layer peeled off from the insulator and heated, and it is obtained as a temperature of a peak appearing next to a first endothermic peak (that is, a second endothermic peak) which is indicative of a sag point. The dilatometric softening point of the glaze layer formed in the surface of the insulator can be also estimated from a value obtained with a glass sample which is prepared by compounding raw materials so as to give substantially the same composition as the glaze layer under analysis, melting the composition and rapidly cooling.